This drama is just wonderful.
The story is so unique and special that you wonder if you'll ever watch a kdrama like this again.
I watched all the episodes in one streak, from morning until late at night.  I just couldn't stop.
When I started this, I had no idea, no clue what this story was about. I was so... mindfucked. I had difficulties to link things and only afterwards, some puzzles were solved. But the longer you watch, the more you get into the characters and the story, the more you understand.
This drama gives so much tension and whenever you think you found a puzzle, it turns out differently. Where you had hope in one episode, it got shattered in another. And where you don't expect it, you get the "aha" feeling.
Seriously, this kdrama is my top 1 favourite.
Don't expect too much romance. Call it more... bromance. The love between two brothers. This drama focuses on the tension, the link between two worlds and the story that plays between the brothers. The directors didn't want to make a cheesy romantic drama. And I'm glad it's not.
I must add one thing more: you'll cry at the end. Because you didn't want it to end. You'll go crazy because there is no season 2. Seriously, I became almost mad xD
But anyway, it's a very special kdrama with a very special story and the cast is as wonderful as the drama itself!

Please check this drama out, It's really worth it.